THAT IS A VERY good question. I have never heard of such a thing! I have had several dogs neutered and know of many others who have had their dogs neutered, and non of us ever had a problem even remotely related to yours.
Generally it only takes a few days to heal, the fact that you are still having trouble leads me to believe the vet may have done something wrong or didn't do the surgery in a sterile environment and your dog has developed some type of chronic infection due to the surgery. Your dog definitly needs to be on antibiotics, and maybe a stronger dose will be required, then initally prescribed. Obviously the first round of antibiotics wasn't administered properly. Either not strong enough or not taken for a long enough period of time.
As well as the antibiotics get some triple antibiotic cream (plus pain reliever) and put a generous amount on the affected area 2 to 3 times aday until you see improvement in the swelling. It Should only take a day or two and you should see a noticeable difference within this time frame.
If things still aren't progressing I would suggest seeing another Vet and getting another opinion. Something is definitly not right and needs to be investigated further, and soon.
Good luck and I hope you get your dog on the mend, (the poor thing has got to be miserable) as this shouldn't have happened in the first place.
